wrote

Am I right in thinking that actually the new St Pancras Thameslink
station is actually to the left of the EMT platforms (but slightly
nearer Euston Road)? I'm sure I saw that on a diagram somewhere -
showing that the Thameslink tunnel actually passes under the Midland
Main Line before the new station. Can anyone confirm that?

The Thameslink tunnel runs under the east side of the original concourse
(about where the Baby Betjeman is now), then ran under the bufferstops end
of platforms 5-2, under the old parcels office and swung to an alignment
under Midland Road as far as Goods Way (and so at an angle to the alignment
of St Pancras station itself). The new station is on the straight stretch
under (the original line of) Midland Road. I would say that this puts the
south end of SPILL south and west of the MML station, and the north end
under the MML station. At one time there was a signalbox on what is now the
Thameslink line, reached by a spiral staircase from the bottom of the cab
ramp on the west side.

Peter